Warm-up of fuel cell power plant

ABSTRACT

A fuel cell ( 1 ) comprises an anode ( 9 A) and cathode ( 9 B) disposed on either side of a solid polymer electrolyte membrane ( 8 A), and power is generated by supplying hydrogen to the anode ( 9 A) and supplying hydrogen to the cathode ( 9 B). When the temperature of the fuel cell ( 1 ) is below 0° C., water in the cell freezes. When a power plant using the fuel cell ( 1 ) is warmed up under the temperature less than 0° C., the anode ( 9 A) is connected to the positive electrode of a secondary battery ( 13 ) and the cathode ( 9 B) is connected to the negative electrode of the secondary battery ( 13 ) to electrolyze frozen water in the cell, thereby thawing the frozen water using the heat accompanying the electrolysis.

FIELD OF THE INVENTION

[0001] This invention relates to warm up of a fuel cell power plantbelow freezing point.

BACKGROUND OF THE INVENTION

[0002] In a polymer electrolyte fuel cell (PEFC), hydrogen ions (H+)pass through a solid polymer electrolyte membrane, so the electrolytemembrane must constantly be kept in a wet state. A fuel cell power plantusing a fuel cell of this type operates while supplying water to theelectrolyte membrane. Therefore, the fuel cell contains a considerableamount of water, and if the power plant is left below freezing in thestationary state, the water in the fuel cell including the water in theelectrolyte membrane will freeze. Consequently, in order to start thefuel cell power plant below freezing point, the ice in the fuel cellmust first be thawed.

[0003] In this regard, Tokkai 2000-315514 published by the JapanesePatent Office in 2000 discloses an apparatus wherein high temperaturegas from outside is supplied to a gas passage in the fuel cell to thawthe ice in the cell.

SUMMARY OF THE INVENTION

[0004] The high temperature gas supplied from the thawing apparatus isdelivered from a pipe outside the fuel cell into a passage formed in aseparator of the fuel cell, and reaches a membrane electrode assembly(MEA) comprising an electrolyte membrane, and electrodes which containcatalyst. However, a considerable amount of heat is lost in the pipe orpassage in the separator before reaching the MEA. This means that theheating efficiency of this apparatus for heating the MEA is poor. Inparticular, in a very low temperature environment of −20 degreescentigrade (° C.) or below, a long time is required until the ice in theMEA is completely thawed, and until the power plant can generate power.

[0005] It is therefore an object of this invention to shorten the timerequired to thaw the ice in an MEA below freezing point.

[0006] In order to achieve the above object, this invention provides amethod of thawing frozen water in a fuel cell, applied to the start-upof a fuel cell power plant comprising a fuel cell stack comprisingplural laminated fuel cells each of which has an anode and cathode oneither side of an electrolyte membrane. The method comprises detecting atemperature of a fuel cell, and thawing the frozen water when thetemperature is less than a freezing point, by a heat generated byapplying a direct current voltage between the anode and cathode to causethe frozen water to undergo electrolysis.

[0007] The details as well as other features and advantages of thisinvention are set forth in the remainder of the specification and areshown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a schematic diagram of a fuel cell power plant applyingthe thawing method of this invention.

[0009]FIG. 2 is a diagram analyzing the production of heat due tochemical reaction in a fuel cell according to the thawing method of thisinvention.

[0010]FIGS. 3A and 3B, are timing charts showing an applied voltage andtemperature variation of the fuel cell according to the thawing methodof this invention.

[0011]FIG. 4 is a schematic cross-sectional view of a three-phaseboundary layer of the fuel cell in the frozen state.

[0012]FIGS. 5A and 5B are timing charts describing a voltage applicationpattern in a thawing method according to a second embodiment of thisinvention.

[0013] FIGS. 6A-6C are timing charts describing variations of fuel celltemperature, applied voltage and power consumption amount according tothe thawing method of the second embodiment of this invention.

[0014]FIG. 7 is an electrical and a signal circuit diagram of a fuelcell power plant applying a thawing method according to a thirdembodiment of this invention.

[0015]FIG. 8 is a timing chart describing a voltage application patternin the thawing method according to the third embodiment of thisinvention.

[0016] FIGS. 9A-9C are timing charts describing variations of fuel celltemperature, applied voltage and power consumption amount according tothe thawing method of the third embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Referring to FIG. 1 of the drawings, a fuel cell power plantcomprises a fuel cell stack 2 comprising a laminate of a polymerelectrolyte fuel cell 1.

[0018] The fuel cell 1 comprises a membrane electrode assembly (MEA) 8and separators 11A, 11B.

[0019] The membrane electrode assembly 8 comprises a solid polymerelectrolyte membrane 8A, and an anode 9A and cathode 9B in pressurecontact on each side of the membrane 8A.

[0020] The electrolyte membrane 8A comprises a perfluorosulfonic acidion-exchange membrane such as Nafion 112 manufactured by Dupont Inc.

[0021] Referring to FIG. 2, the anode 9A and cathode 9B comprise a gasdiffusion layer (GDL) 21 formed of carbon paper, and a three-phaseboundary layer 22. The three-phase boundary layer 22 comprises themixture of a platinum catalyst on carbon black, and the sameperfluorosulfonic acid material used in the electrolyte membrane 8A. Themixture is coated on GDL 21. The three-phase boundary 22 has a threephase boundary between a gas comprising hydrogen or oxygen, anelectrolyte, and the catalyst.

[0022] Referring again to FIG. 1, the separator 11A and separator 11Bare comprised of an electrically-conducting material. In the separator11A, a passage 10A for hydrogen-rich gas is formed adjacent to the anode9A. In the separator 11B, a passage 10B for oxygen is formed adjacent tothe cathode 9B.

[0023] Hydrogen-rich gas is supplied to the passage 10A from a hydrogencylinder 5 via a pressure regulating valve 6. Oxygen is supplied as airto the passage 10B from a compressor 3 via a pressure regulating valve4.

[0024] Hydrogen which has undergone an electrochemical reaction in theanode 9A becomes hydrogen ions, passes through the electrolyte membrane8A, and combines with oxygen in the cathode 9B to form water vapor.

[0025] The water vapor is mainly emitted to the atmosphere as exhaustgas from the cathode 9B. Part of the water vapor diffused to the anode9A via the electrolyte membrane 8A, and is emitted to the atmospherefrom the anode 9A.

[0026] As a large amount of hydrogen is supplied to the anode 9A, theexhaust gas from the anode 9A contains a considerable amount of hydrogenin addition to the water vapor. Therefore, an ejector 7 recycles thisexhaust gas into the flow of hydrogen from the pressure regulating valve6 to the passage 10 as anode effluent, and thus makes use of hydrogenefficiently.

[0027] The laminated fuel cells 1 are electrically connected in seriesbetween electrodes 18A, 18B of the stack 2. The anode 9A is electricallyconnected to the cathode 9B of the adjacent fuel cell 1 via theseparator 11A, and the separator 11B of the adjacent fuel cell 1. Theelectrode 18A is connected to a switch 12.

[0028] The switch 12 is connected to a motor 15 as a load via aninverter 17, and to a secondary battery 13 via a constant current supplycircuit 14, respectively, and connects the motor 15 and secondarybattery 13 selectively to the electrode 18A according to the switchingposition. The electrode 18B is connected to an earth wire of the motor15 and the negative electrode of the secondary battery 13.

[0029] The switch 12, inverter 17 and constant current supply circuit 14are respectively controlled according to signals output from acontroller 16. The controller 16 comprises a microcomputer comprising acentral processing unit (CPU), read-only memory (ROM), random accessmemory (RAM) and input/output interface (I/O interface). The controllermay also comprise plural microcomputers.

[0030] To perform the above control, a temperature signal is input tothe controller 16 from a temperature sensor 19 which detects thetemperature of the fuel cell stack 2.

[0031] In FIG. 1, the solid lines show the gas flow, the dash-and-dotlines show the electricity flow, and the broken lines show the signalflow.

[0032] When the power plant starts, if the temperature detected by thetemperature sensor 19 is below freezing point, the controller 16controls the switch 12 to connect the positive electrode of thesecondary battery 13 to the electrode 18A of the fuel cell stack 2. As aresult, in the fuel cells 1, a direct current flows from the anode 9A tothe cathode 9B.

[0033] Herein, if the power generating surface area of the electrolytemembrane 8A of the fuel cell 1 is 25cm², and a current of 1 ampere persquare centimeter (A/cm²) is to be passed to the electrolyte membrane8A, a current of 25A must be passed to the fuel cell stack 2. Theconstant current supply circuit 14 increases or decreases the voltage ofthe secondary battery 13 so that the required current flows.

[0034] Referring again to FIG. 2, when the MEA 8 is frozen, and apositive voltage is applied to the anode 9A and a negative voltage isapplied to the cathode 9B, oxygen is produced at the anode 9A andhydrogen is produced at the cathode 9B due to electrolysis of the frozenwater in the MEA 8.

[0035] When a voltage above a certain level is applied to the frozenfuel cell 1, electrolysis of water takes place according to thefollowing equation (1): $\begin{matrix}{\left. {H_{2}O}\rightarrow{{\frac{1}{2}O_{2}} + {2H^{+}} + {2e^{-}}} \right.\left. {{2H^{+}} + {2e^{-}}}\rightarrow H_{2} \right.} & (1)\end{matrix}$

[0036] In this state, hydrogen is supplied to the anode 9A from thehydrogen cylinder 5, air is supplied to the cathode 9B from thecompressor 3, and at the anode 9A, the oxygen produced and hydrogensupplied combine to form water. Also at the cathode 9B, the hydrogenproduced and oxygen in the supplied air combine to form water. Thecombination of hydrogen and oxygen is accompanied by the release ofheat. Therefore, the ice in the fuel cell stack 2 can be thawed usingthis heat.

[0037] The case will now be considered when the power plant is undertemperature conditions of −30° C. Under these temperature conditions,the ion conducting efficiency of the electrolyte membrane 8A isapproximately 4 millisiemens per centimeter (mS/cm), so the voltage dropdue to the electrolyte membrane is approximately 0.7 volts (V) at 1A/cm² of current density. The catalyst activation voltage isapproximately 0.3V, and the sum of the voltage drop due to the contactresistance between the GDL 21 and separator 11A, and the voltage dropdue to the contact resistance between the GDL 21 and separator 11B, isapproximately 0.2V.

[0038] The voltage required for electrolysis of water is approximately1.2V which can be calculated using the Nernst equation on theequilibrium potential required to balance an ion concentrationdifference across a membrane, shown by the following equation (2).$\begin{matrix}{E_{r} = {{E_{O2} - E_{H2}} = {E_{0} + {{\frac{2.3 \cdot R \cdot T}{2 \cdot F} \cdot \log}\frac{\sqrt{p_{o2}} \cdot p_{H2}}{p_{H2O}}}}}} & (2)\end{matrix}$

[0039] where,

[0040] E_(r)=equilibrium potential,

[0041] E_(O2)=anode potential,

[0042] E_(H2)=cathode potential,

[0043] E₀=standard potential=+1.2V under atmospheric pressure,

[0044] R=gas constant,

[0045] T=temperature, and

[0046] P_(O2), P_(H2), P_(H20)=partial pressures.

[0047] If the total loss of 1.2V due to voltage drop is added to this1.2V, a voltage of approximately 2.4V must be applied between theelectrodes in order to perform electrolysis of water in the fuel cell 1at −30° C. Of this, 1.2V corresponding to an overpotential excluding thepart used for electrolysis is converted to heat, and can be used forthawing ice. When a current of 1A/cm² is passed to the fuel cell 1, thiscorresponds to a heat release of 1.2 watts per square centimeter(W/cm²). By using this released heat to thaw ice, the ice in the fuelcell stack 2 can be thawed in a short time even in a very coldenvironment of −30° C.

[0048] As described above, the hydrogen produced by electrolysis ofwater combines with oxygen in the air supplied from the compressor 3,and the oxygen produced by electrolysis of water combines with hydrogensupplied from the hydrogen cylinder 5, which respectively generate heatof reaction. This generated heat further promotes the thawing of ice inthe fuel cell stack 2. However, immediately after applying a voltage,the GDL 21 is frozen, so a combination reaction hardly occurs betweenthis hydrogen and oxygen, and the generation amount of reaction heat dueto combination between hydrogen and oxygen also increases as thawingproceeds.

[0049] Next, the heat produced by applying a voltage between theelectrodes of the fuel cell 1 will be described in further detail.

[0050] The Joule heat produced in the electrolyte membrane 8A, GDL21 andseparators 11A, 11B may be expressed as i²·R. Herein, i is the currentdensity, and R is the electrical resistance of the electrolyte membrane8A, GDL21 and separators 11A, 11B.

[0051] At the contact surface between the GDL21 and separators 11A, 11B,the surface area, heat is generated according to these contactresistances. In the vicinity of the catalyst of the GDL 21, most of theenergy corresponding to the overpotential required to promote thechemical reaction of equation (1) is converted to heat. All this heat isuseful in thawing ice.

[0052] Further, due to combination between the oxygen produced at theanode 9A by electrolysis of water and the hydrogen supplied from thehydrogen cylinder 5, and combination between the hydrogen produced atthe cathode 9B by electrolysis of water and oxygen supplied as air fromthe compressor 3, heat of reaction is generated as described earlier.

[0053] Referring to FIGS. 3A, 3B, the results of thawing experimentsperformed on the fuel cell 1 by the Inventor will now be described.

[0054] When a voltage is applied between the electrodes of the fuel cell1 frozen at −30° C., the temperature of the fuel cell 1 first increasesdue to the Joule heat produced in the fuel cell 1 due to application ofthe voltage. This temperature rise temporarily stops at −20° C. Theinterruption of the temperature rise shows that water which is partiallybound in the electrolyte membrane 8A, is thawing.

[0055] Herein, the water in the electrolyte membrane 8A will bedescribed. In the electrolyte membrane 8A, water which is bound tosulfonic acid groups and which is not frozen, free water not bound tosulfonic acid groups and frozen at approximately 0° C. and partiallybound water frozen at −20° C., are present. The weight concentration ofthe mixed water which is the sum of the above, is approximately tentimes the weight concentration of the sulfonic acid groups.

[0056]FIG. 4 shows the state inside the three-phase boundary layer 22 at−30° C. In the three-phase boundary layer 22, an electrochemicalreaction occurs at the three phase boundary of electrolyte made ofperfluorosulfonic acid, Pt catalyst and oxygen gas or hydrogen gas.However, at −30° C., water inside the three-phase boundary layer 22freezes, and blocks penetration of gas into the catalyst from the GDL21. In the Figure, only part of the three-phase boundary layer 22 isshown as blocked, but in practice effectively the whole surface isblocked. In this state, the fuel cell 1 cannot generate power, and theoxygen and hydrogen produced at the catalyst by electrolysis, cannot mixwith the hydrogen and oxygen supplied from the GDL 21.

[0057] Referring again to FIGS. 3A, 3B, the reason why the temperaturerise of the fuel cell 1 temporarily stops at −20° C., is that thepartially bound water in the electrolyte membrane 8A consumes heat forthawing.

[0058] The thawing of the partially bound water is complete atapproximately 50 seconds from when the voltage starts to be applied, andthe temperature of the fuel cell 1 again rises. When it reaches 0° C.,the temperature rise again temporarily stops due to the thawing offrozen free water. When the thawing of free water is complete, theoxygen produced by electrolysis can combine with hydrogen supplied tothe passage 10A from the hydrogen cylinder 5, and the hydrogen producedby electrolysis can combine with oxygen supplied to the passage 10B fromthe compressor 3. The heat of the reaction accompanying the combinationof this hydrogen and oxygen, accelerates the temperature rise of thefuel cell 1. In FIG. 3A, the sharp slope of the temperature rise of thefuel cell 1 after all the free water has thawed, is due to this heat ofthe reaction.

[0059] On the other hand, as the temperature of the fuel cell 1 rises,the voltage applied between the anode 9A and cathode 9B decreases. Asdescribed above, when the voltage starts to be applied, it is 2.4V, butwhen the temperature is +20° C., the electrical conductivity of theelectrolyte membrane 8A is 30 mS/cm. As a result, the voltage drop dueto the electrolyte membrane 8A is approximately 0. 1V, and the appliedvoltage between the electrodes is 1.8V, which is sufficient. If acurrent of 1A/cm² is passed through the fuel cell 1 in this state, theemitted heat amount corresponds to approximately 0.6W/cm².

[0060] In this experiment, it was found that about 2 minutes after thevoltage starts to be applied, thawing of ice in the fuel cell 1 wascomplete, and the fuel cell 1 could generate power. The time required tothaw the fuel cell 1 can be further shortened by heating or humidifyingthe hydrogen supplied to the passage 10A and the air supplied to thepassage 10B.

[0061] The aforesaid embodiment relates to a fuel cell power plantcomprising a single fuel cell stack. In a power plant comprising pluralfuel cell stacks, the ice in one of the fuel cell stacks is first thawedby the above method, and the other fuel cell stacks are then thawedusing the output current of the fuel cell stack which is now able togenerate power due to thawing. If this is done, the load on thesecondary battery 13 can be mitigated.

[0062] Next, a second embodiment of this invention will be described.

[0063] This embodiment concerns the method of applying a voltage to theelectrodes 18A, 18B of the fuel cell stack 2. The construction of thehardware is identical to that of the first embodiment, but the positiveelectrode and negative. electrode of the secondary battery 13 arereversed. Specifically, the negative electrode of the secondary battery13 is connected to the electrode 18A, and the positive electrode of thesecondary battery 13 is connected to the electrode 18B.

[0064] Even if hydrogen or oxygen is supplied to the fuel cell 1 at −30°C., the water in the MEA 8 is frozen, so hydrogen or oxygen from the GDL21 cannot reach the catalyst in the three-phase boundary layer 22, andelectrochemical reactions in the layer do not occur.

[0065] If a voltage is applied to the electrodes 18A, 18B in the abovestate as described above, hydrogen is produced at the anode 9A andoxygen is produced in the cathode 9B. 60 seconds after the voltagestarts to be applied, thawing of the partially bound water in theelectrolyte membrane 8A is complete, as shown in FIG. 3A. At this stage,the switch 12 is operated to change-over the fuel cell stack 2 to theload. Hydrogen is then supplied from the passage 10A to the anode 9A andoxygen is supplied from the passage 10B to the cathode 9B.

[0066] At this point, thawing of the partially bound water in theelectrolyte membrane 8A is complete, so part of the hydrogen or oxygenreaches the catalyst in the three-phase boundary layer 22, and hydrogenor oxygen produced by electrolysis remains at anode or cathode. As aresult, electrochemical reactions at the three-phase boundary can takeplace and power generation begins. The power generation takes placeaccording to equation (3), which is the reverse of equation (1).$\begin{matrix}{\left. {{\frac{1}{2}O_{2}} + {2H^{+}} + {2e^{-}}}\rightarrow{H_{2}O} \right.\text{}\left. H_{2}\rightarrow{{2H^{+}} + {2e^{-}}} \right.} & (3)\end{matrix}$

[0067] In this power-generating process, heat is produced due to thevoltage drop caused by membrane resistance to movement of hydrogen ionsin the electrolyte membrane 8A, the voltage drop caused by contactresistance between the GDL 21 and separators 11A, 11B, and the voltagedrop due to catalyst activation resistance, respectively. The motor 15is connected to the fuel cell stack 2, and the hydrogen supply amountand an inverter 17 are controlled to produce power so that theresistance of the motor 15 is 25 milliohms (mΩ) and the current densityof the fuel cell stack 2 is cm². However, if the current density is1A/cm² and the potential difference between the electrodes of the fuelcell 1 is less than 0.4V, adjustment is made in so that it is 0.4V toprevent damage to the fuel cell 1. The heat released. due to theoverpotential at this time is within the range of 0.6-1.2W/cm² dependingon the temperature of the fuel cell 1.

[0068] In this embodiment, the voltage-applying process andpower-generating process are repeatedly performed as shown in FIGS. 5Aand 5B, below 0° C. As a result, as in the first embodiment, the powerconsumed due to applying the voltage can be reduced compared to the casewhere the voltage is simply applied continuously, and the warm-upefficiency of the fuel cell stack can be increased.

[0069] In FIGS. 5A and 5B, voltage-applying time periods T1, T3, T5 andpower-generating time periods T2, T4, T6 are all set to be equal. Thissetting can also be varied according to the thawing conditions. Forexample, if the voltage-applying time period is set to be longer thanthe power-generating time period the hydrogen and oxygen required forpower generation can be mainly obtained from electrolysis of water.

[0070] Alternatively, the frequency of change-over between thevoltage-applying process and power-generating process can be increasedaccording to the elapsed time. In other words, it is also preferable toset T1>T3>T5, T2>T4>T6.

[0071] Referring to FIGS. 6A-6C, the Inventor carried out an experimentwherein the voltage-applying process and power-generating process werealternated with T1=60 seconds, T2=50 seconds, T3=40 seconds, T4 =50seconds, T5=30 seconds, and the system then changed over to ordinarypower generation.

[0072] At −30° C., when the voltage is applied for 60 seconds betweenthe anode and cathode of the fuel cell 1, Joule heat is produced in thecell, and the temperature of the cell rises. At −20° C., the temperaturerise temporarily stops due to thawing of the partially bound water inthe electrolyte membrane 8A as described above.

[0073] Thawing of the partially bound water terminates approximately 50seconds after the voltage starts to be applied, and the temperature ofthe fuel cell 1 then rises again. After a voltage has been applied for60 seconds, the system changes over to the power-generating process. Inthe power-generating process, hydrogen is supplied from the hydrogencylinder 5 to the passage 10A, and air is supplied from the compressor 3to the passage 10B. Inside the fuel cell 1, power is generated usingthis hydrogen and the oxygen contained in the air, and the temperatureof the fuel cell 1 rises due to the heat accompanying power generation.Also, oxygen generated at the anode 18A in the voltage-applying processcombines with hydrogen in the passage 10A, and hydrogen generated at thecathode 18B combines with oxygen in the passage 10B. The heat due tothese combinations promotes temperature rise of the fuel cell 1. Theelectrical energy generated in the power-generating process is allconsumed as heat. In this power-generating process, the power of thesecondary battery 13 is of course not consumed.

[0074] After the power-generating process has continued for 50 seconds,they voltage-applying process is again performed for 40 seconds.Approximately 15 seconds after the change-over, the temperature of thefuel cell 1 reaches 0° C. At 0° C., the heat produced by is used aslatent heat of liquefaction to thaw the free water, and the temperaturerise of the fuel cell 1 temporarily stops. Subsequently, after thevoltage-applying process has terminated, the power-generating processlasting for 50 seconds and the voltage-applying process lasting for 30seconds are performed alternately.

[0075] In this way, the free water in the MEA 8 is all thawed after 250seconds has elapsed from when the thawing operation was begun at −30° C.

[0076] The power consumed during the thawing operation is generated onlyduring the voltage-applying process and not during the power-generatingprocess, as shown in FIG. 6C. Also, the applied voltage starts from2.4V, decreases together with temperature rise of the fuel cell 1, andfalls to 1.8V at 0° C.

[0077] According to this experiment, the thawing of the fuel cell 1 wascompleted in approximately 250 seconds after the voltage starts to beapplied. The power consumption amount from when the voltage starts to beapplied to completion of thawing was 100 Joules /cm².

[0078] Next, referring to FIGS. 7, 8 and FIGS. 9A-9C, a third embodimentof this invention will be described.

[0079] First, referring to FIGS. 7, a power plant according to thisembodiment comprises another switch 20 between the switch 12 of thefirst embodiment and the constant current supply circuit 14. The switch12 connects the electrode 18A of the fuel cell stack 2 selectively toone of a contact 12A leading to the constant current supply circuit 14,and a contact 12B leading to the inverter 17. The switch 20 connects oneof the positive electrode of the secondary battery 13 via the constantcurrent supply circuit 14 and the negative electrode of the secondarybattery 13 to the contact 12A, and the other to the electrode 18B of thefuel cell stack 2. The change-over of the switches 12, 20 is controlledby the controller 16. The remaining features of the hardware of thepower plant are identical to those of the first embodiment shown in FIG.1.

[0080] According to this embodiment, when the contact 12A of the switch12 is connected to the electrode 18A, the controller 16 operates theswitch 20 change-over between a state wherein the positive electrode ofthe secondary battery 13 is connected to the electrode 18A via theconstant current supply circuit 14 and the negative electrode of thesecondary battery 13 is connected to the electrode 18B, and a statewherein the negative electrode of the secondary battery 13 is connectedto the electrode 18A and the positive electrode of the secondary battery13 is connected to the electrode 18B via the constant current supplycircuit 14.

[0081] When the power plant starts up, if the temperature detected bythe temperature sensor 19 is below freezing point, the controller 16operates the switch 12 to connect the contact 12A to the electrode 18A,and operates the switch 20 to connect the negative electrode of thesecondary battery 13 to the electrode 18A, and connect the positiveelectrode of the secondary battery 13 to the electrode 18B via theconstant current supply circuit 14. As a result, frozen water in the MEA8 of the fuel cell 1 is electrolyzed, hydrogen is produced at the anode9A and oxygen is produced at the cathode 9B of each of the fuel cells 1.In the following description, this process is referred to as ahydrogen/oxygen generating process. In this process, the controller 16controls the constant current supply circuit 14 so that the currentdensity in the electrolyte membrane 8A is 1A/cm², as in the firstembodiment.

[0082] As described above, the ion conductivity of the electrolytemembrane 8A is 4 mS/cm² at −30° C., and 30 mS/cm² at −20° C. The voltagedrop in the electrolyte membrane 8A when voltage is applied is 0.7V at−30° C., and 0.1V at −20° C. As the catalyst activation voltage isapproximately 0.3V, the voltage drop in the GDL 21 and separator 11A isapproximately 0.2V, and the voltage required for electrolysis of wateris 1.2V, it is necessary to apply a voltage within the range from 2.4Vto 1.8V according to the temperature in order to perform electrolysis ofwater in the fuel cell 1. Of this voltage, an overpotential of 0.6-1.2Vis used for generating heat. As the current density is 1A/cm², the powerconsumption amount is 0.6-1.2W/cm².

[0083] Next, the controller 16 changes over the switch 20 so that thepositive electrode of the secondary battery 13 is connected to theelectrode 18A via the constant current supply circuit 14 and thenegative electrode of the secondary battery 13 is connected to theelectrode 18B. As a result, oxygen is produced at the anode 9A andhydrogen is produced at the cathode 9B of the fuel cell 1, as in thefirst embodiment. In the following description, this process is referredto as an oxygen/hydrogen generating process. In this process also, thecontroller 16 controls the constant current supply circuit 14 in thesame way as in the hydrogen/oxygen generation process.

[0084] The oxygen produced at the anode 9A in the oxygen/hydrogengeneration process combines with hydrogen produced at the anode 9A inthe hydrogen/oxygen generating process to form water. The hydrogenproduced at the cathode 9B in the oxygen/hydrogen generation processcombines with oxygen produced at the cathode 9B in the hydrogen/oxygengenerating process to form water. Heat of reaction is generated togetherwith these water-forming reactions.

[0085] The controller 16 changes over the switch 20 to alternate betweenthe hydrogen/oxygen generating process and the oxygen/hydrogengenerating process as shown in FIG. 8, and the thawing of ice in the MEA8 is promoted using the reaction heat of water forming. In FIG. 8, thevoltage application state wherein the anode 9A is at a higher potentialthan the cathode 9B is expressed as a positive voltage, and the voltageapplication state wherein the cathode 9B is at a higher potential thanthe anode 9A is expressed as a negative voltage.

[0086] The hydrogen/oxygen generating process continuation time T1 andoxygen/hydrogen generation process continuation time T2 are effectivelyidentical, but considering that hydrogen is supplied to the anode 9Afrom the hydrogen cylinder 5, and oxygen is supplied to the cathode 9Bas air from the compressor 3, T1 may be set less than T2.

[0087] As the thawing of the MEA 8 proceeds, hydrogen and oxygensupplied to the fuel cell 1 from outside reaches the catalyst moreeasily. In this regard, It is preferred to vary the ratio of T1 and T2according to the progress of the thawing. Specifically, the time T1 ofthe hydrogen/oxygen process is shortened as thawing proceeds. Thisdecreases the hydrogen production amount at the anode 9A and the oxygenproduction amount at the cathode 9B, and the decreases are compensatedby hydrogen supplied from the hydrogen cylinder 5 and oxygen in the airsupplied from the compressor 3. In this way, the thawing time isshortened.

[0088] Referring to FIGS. 9A-9C, the results of the thawing experimentrelating to this embodiment performed by the Inventor, will now bedescribed.

[0089] At −30° C., as in the first and second embodiments, thehydrogen/oxygen process wherein the negative electrode of the secondarybattery 13 was connected to the electrode 18A, and the positiveelectrode of the secondary battery 13 was connected to the electrode 18Bvia the constant current supply circuit 14, was performed for 20seconds. Next, the oxygen/hydrogen generating process wherein thepositive electrode of the secondary battery 13 was connected to theelectrode 18A via the constant current supply circuit 14, and thenegative electrode of the secondary battery 13 was connected to theelectrode 18B, was performed for 40 seconds. Next, the hydrogen/oxygenprocess was performed for 30 seconds, and the oxygen/hydrogen generatingprocess was performed for 40 seconds. During the experiment, hydrogenwas supplied to the anode 9A from outside, and air was supplied to thecathode 9B from outside.

[0090] In the first embodiment, in the initial stage of the thawing,oxygen produced at the anode 9A and hydrogen supplied from outside, andhydrogen produced at the cathode 9B and oxygen supplied as air fromoutside, are respectively blocked by the ice in the three-phase boundarylayer 22 so they cannot combine. On the other hand, in this embodiment,hydrogen produced at the anode 9A combines with oxygen produced at theanode 9A, and oxygen produced at the cathode 9B combines with hydrogenproduced at the cathode 9B, even in the initial stage of the thawing.Therefore, the reaction heat due to combination can be fully used fromthe initial stage of thawing. Also, in the latter half of thawing, byrespectively compensating the hydrogen production amount of the anode 9Aand oxygen production amount at the cathode 9B with hydrogen and oxygensupplied from outside, the hydrogen/oxygen process can be shortened.Further, in this embodiment, the temperature variation trends of thefuel cell 1 identical to those of the first embodiment. However, whereasthe time from beginning voltage application to completion of thawing is250 seconds in the first embodiment, it is considerably shortened to 90seconds in this embodiment.

[0091] The contents of Tokugan 2001-396579 and Tokugan 2001-396587 bothof which were filed on Dec. 27, 2001 in Japan, are hereby incorporatedby reference.

[0092] Although the invention has been described above by reference tocertain embodiments of the invention, the invention is not limited tothe embodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings.

INDUSTRIAL FIELD OF APPLICATION

[0093] As described above, according to this invention, frozen water inthe fuel cell is thawed using the electrolysis of water, and the fuelcell can be put into a power generating state with high efficiency andwithout the need for a special thawing device such as a heater orblower. Therefore, a particularly desirable effect is obtained when thisinvention is applied to a fuel cell power plant for vehicle mounting.

[0094] The embodiments of this invention in which an exclusive propertyor privilege is claimed are defined as follows:

1. A method of thawing frozen water in a fuel cell (1), applied to thestart-up of a fuel cell power plant comprising a fuel cell stack (2)comprising plural laminated fuel cells (1) each of which has an anode(9A) and cathode (9B) on either side of an electrolyte membrane (8A),the method comprising: detecting a temperature of a fuel cell (1); andthawing the frozen water when the temperature is less than a freezingpoint, by a heat generated by applying a direct current voltage betweenthe anode (9A) and cathode (9B) to cause the frozen water to undergoelectrolysis.
 2. The thawing method as defined in claim 1, wherein thepower plant comprises a secondary battery (13) having a positiveelectrode and a negative electrode to supply the direct current voltage,and the method further comprises connecting the anode (9A) to thepositive electrode and the cathode (9B) to the negative electrode tocause the frozen water to undergo the electrolysis.
 3. The thawingmethod as defined in claim 2, wherein the method further comprisessupplying hydrogen to the anode (9A) so as to form water by combiningwith oxygen generated at the anode (9A) by the electrolysis of thefrozen water.
 4. The thawing method as defined in claim 2, wherein themethod further comprises supplying oxygen to the cathode (9B) so as toform water by combining with hydrogen generated at the cathode (9B) bythe electrolysis of the frozen water.
 5. The thawing method as definedin any one of claim 1 through claim 4, wherein the voltage appliedbetween the anode (9A) and cathode (9B) is set to a voltagecorresponding to the sum of a voltage required to electrolyze water, anda voltage drop corresponding to an electrical resistance between theanode (9A) and cathode (9B).
 6. The thawing method as defined in any oneof claim 1 through claim 4, wherein the power plant further comprises asecond fuel cell stack (2), and the method further comprises thawingfrozen water in the fuel cell stack (2) by using a power generated bythe first fuel cell stack (2) after thawing of the frozen water in thefirst fuel cell stack (2) has completed.
 7. The thawing method asdefined in any one of claim 1 through claim 4, wherein the fuel cell (1)generates power using hydrogen supplied to the anode (9A) and oxygensupplied to the cathode (9B), and the method further comprisesalternately repeating a direct current voltage-applying process whichapplies a direct current voltage between the anode (9A) and cathode (9B)and a power-generating process which supplies hydrogen to the anode (9A)while supplying oxygen to the cathode (9B).
 8. The thawing method asdefined in claim 7, wherein an alternation frequency between the directcurrent voltage-applying process and the power-generating process, isincreased with an elapsed time.
 9. The thawing method as defined inclaim 7, wherein the power plant comprises a secondary battery (13)having a positive electrode and a negative electrode to supply thedirect current voltage, and the method further comprises connecting theanode (9A) to the negative electrode and the cathode (9B) to thepositive electrode to cause the frozen water to undergo theelectrolysis.
 10. The thawing method as defined in claim 1, wherein thepower plant comprises a secondary battery (13) having a positiveelectrode and a negative electrode to supply the direct current voltage,and the method further comprises repeating an oxygen/hydrogen generationprocess wherein the anode (9A) is connected to the positive electrodeand the cathode (9B) is connected to the negative electrode so as tocause the frozen water to undergo electrolysis to generate oxygen at theanode (9A) and generate hydrogen at the cathode (9B), and ahydrogen/oxygen generating process wherein the anode (9A) is connectedto the negative electrode and the cathode is connected to the positiveelectrode so as to cause the frozen water to undergo electrolysis togenerate hydrogen at the anode (9A) and generate oxygen at the cathode(9B).
 11. The thawing method as defined in claim 10, wherein the methodfurther comprises supplying hydrogen to the anode (9A) while supplyingoxygen to the cathode (9B) so as to cause a time length of thehydrogen/oxygen generating process to be shorter than a time length ofthe oxygen/hydrogen generating process.
 12. The thawing method asdefined in claim 10 or claim 11, wherein the method further comprisessupplying hydrogen to the anode (9A) while supplying oxygen to thecathode (9B) and varying a time length of the hydrogen/oxygen generatingprocess according to an elapsed time.